1. Field of the Invention
The present invention relates to protection methods, protection circuits and protective devices for rechargeable batteries, to a power tool and charger adapted to provide protection for cells of an attached battery pack, and to a battery pack including protection control therein, each protecting the battery back against various potential fault conditions.
2. Description of Related Art
Over the past few years, lithium-ion (Li-ion) batteries have begun replacing nickel-cadmium (NiCd), nickel-metal-hydride (NiMH), and lead-acid batteries in low-voltage, portable electronic devices such as notebook-type personal computers. As compared to NiCd and NiMH batteries, Li-ion batteries are lighter but have a larger capacity per unit volume. For this reason, the Li-ion batteries have been typically suitable to low-voltage devices that are preferably light and which are required to endure continuous use for a long time. In an over-discharged state, however, the Li-ion batteries deteriorate rapidly, thus Li-ion batteries require over-discharge protection.
A battery pack used in a portable electronic device typically has a plurality of battery cells connected in series. The maximum number of battery cells connected in series in one battery pack is determined by the output voltage of the battery pack. For instance, the typical output voltage of one NiCd battery cell or one NiMH battery cell is 1.2 V. Assuming that an 18V output voltage from a battery pack is suitable for most general purpose electronic devices, the maximum number of NiCd or NiMH battery cells connected in series in the battery pack is 15. On the other hand, the typical output voltage of one Li-ion battery cell is approximately 3.6 V. Accordingly, the maximum number of Li-ion battery cells connected in series in one fictional 18V Li-ion battery pack would be 5.
Unlike a NiCd battery pack and a NiMH battery pack, the Li-ion battery pack may include functionality to protect against fault conditions inside and outside the Li-ion battery pack. This prevents cells in the Li-ion battery pack from deteriorating and shortening useful life of the pack. For instance, if a fault condition such as short-circuiting occurs inside or outside the Li-ion battery, a fuse may be provided to cut off an over-discharging current or an overcharging current, if the discharging current or charging current becomes larger than a given current level.
Currently, protection circuits in battery packs such as Li-ion battery packs are designed primarily for low-voltage portable electronic devices such as notebook-type personal computers, cellular phones, etc., which require voltage generally on the order of 2 to 4 volts. Such devices are characterized by using battery packs composed of cells (such as Li-ion, NiCd, NiMH cells) that provide a maximum output voltage of about 4.2 volts/cell. For Li-ion battery cells, care must be taken to prevent damage from electrical and mechanical stresses, since lithium is a highly reactive substance.
Conventional protection circuits for these low-voltage battery packs may monitor cell voltages to prevent a given cell from over-charging or over-discharging, and may monitor current to keep current from rising too high. Other protection circuits may have one or more temperature inputs to disable current during charge or discharge until the battery pack cools down. Still other protection circuits may be designed to help maintain the balance of charge on the cells, commonly known as equalization circuits. A typical protection circuit may be connected to a given battery cell or group of cells in the battery pack to avoid these situations. For example, a conventional protection circuit may typically include a pair of MOSFET's or other semiconductors that can stop current flow in either direction.
However, much higher voltages than described above are required for higher-power electronic devices such as cordless power tools. Accordingly, higher-power battery packs may be in the process of being developed for cordless power tools. Such “high-power” battery packs may provide higher voltage outputs than conventional NiCd and NiMH battery packs (and substantially higher power than conventional Li-ion packs used for PCs and cell phones), and at a much reduced weight (as compared to conventional NiCd or NiMH battery packs used as power sources in conventional cordless power tools). A characteristic of these battery packs is that the battery packs may exhibit substantially lower impedance characteristics than conventional NiCd, NiMH and/or even the lower power Li-ion packs.
Further, as these battery technologies advance, the introduction of lower impedance chemistries and construction styles to develop secondary batteries generating substantially higher output voltages (of at least 18 V and up, for example) may possibly create several additional protection issues. Battery packs having lower impedance also means that the pack can supply substantially higher current to an attached electronic component, such as a power tool. As current through a motor of the attached power tool increases, demagnetization forces (e.g., the number of armature turns of the motor times the current, ampere-turns) could substantially increase beyond a desired or design limit in the motor. Such undesirable demagnetization could thus potentially burn up the motor.
For example, a lower impedance electrical source could cause damage to a tool's motor when the tool is held at stall condition. During motor stall, the motor and battery impedances are the only mechanisms to limit the current since there is no back-EMF created by the motor. With a lower impedance pack, the currents would be higher. Higher currents through the motor will increase the likelihood of de-magnetization of the permanent magnets within the tool's motor.
Additionally, start-up of the tool could produce excessive starting currents and cause demagnetization of the motor. Thermal overload could also be a result of using a low impedance electrical source in an existing power tool, as the new batteries may be designed to run longer and harder than what the original cordless tool system was designed.
Accordingly, different protection controls may need to be in place to address potential fault conditions that could occur in high power battery packs that are adapted for use with both existing cordless power tools, and developing lines of power tools that are manufactured for use with these higher power battery packs. In particular, protection controls need to be developed to handle fault conditions such as over-charge, over-discharge, over-current, over-temperature and cell imbalance which could occur in one or more cells of a battery pack (such as a Li-ion or NiCd pack), so as to prevent internal or external damage to the pack, an attached device such as a charger or tool or to a user in the vicinity of a pack connected to a charger or tool.